1. Field of the Invention
The present invention concerns a method for representation of flow in a magnetic resonance image.
2. Description of the Prior Art
It is well known that magnetic resonance imaging is sensitive to movement in the examination subject, such as, for example, blood flow in the human body. Due to the better understanding of the effects that occur in the MR imaging and their influence on the MR image, it would be possible to develop imaging techniques that are based on signal changes that are induced by moving spins in order to represent flow in the MR image.
The effects that occur are what are known as time of flight effects and phase effects. The time of flight effects are based on the fact that, in the imaging, new spins flow into the image plane that produce a different signal in the signal readout than the spins that were previously excited in the image plane with a radio-frequency pulse.
The representation of flow effects in the MR image due to the phase effect are based on the fact that excited spins, i.e. spins with transverse magnetization, move along magnetic field gradients, whereas the phase of the moving spins changes dependent on the speed.
The above-described influences on the MR signal of the moving spins can be used in order to generate, for example, MR angiographies. Conventionally, only a conclusion about the level of the flow speed along one spatial direction (also opposite to this spatial direction) was possible. A conclusion about the flow direction that is not limited to one spatial direction has previously not been possible.
Furthermore, color Doppler ultrasound systems are known that are based on the fact that the ultrasound frequency between received and sent wave frequency depends on the relative speed between the transmitter and receiver. In the color Doppler ultrasound method, an overview of the flow speed and the flow direction can be quickly obtained by the color coding of the blood flow.